Manufacturers of internal combustion engines must satisfy customer demands and meet various regulations for reduced emissions and improved fuel economy. One example of a way to improve fuel economy is to operate an engine at an air/fuel ratio that is lean (excess oxygen) of stoichiometry. Examples of lean-burn engines include compression-ignition (diesel) and lean-burn spark-ignition engines. However, while lean-burn engines may have improved fuel economy, the exhaust gas emitted from such an engine, particularly a diesel engine, is a heterogeneous mixture that contains gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and oxides of nitrogen (“NOx”) as well as condensed phase materials (liquids and solids) that constitute particulate matter (“PM”). The commercial application of lean-burn engines has been limited due to a lack of effective methods to remove sufficient NOX from the lean exhaust gas stream before it exits the tail pipe to meet regulations. Thus, efficient reduction of NOX from lean-burn gasoline and diesel exhaust before it exits the tail pipe is important to meet future emission standards and improve vehicle fuel economy.
Several potential exhaust treatment systems have been proposed for vehicle applications. These systems employ various exhaust treatment devices. One such exhaust treatment system employs a urea selective catalyst reduction (SCR) catalyst and a NOX reductant (e.g., urea) that is injected upstream of the catalyst using a generally downstream facing fluid injector. The NOx reductant is converted to ammonia that is used to reduce NOX to N2. Use of urea as a reductant necessitates a urea distribution infrastructure and an on-vehicle monitoring system for this secondary fluid. Such systems require periodic catalyst regeneration involving fuel injection or injection of reductant to regenerate the storage material of the catalyst.
An exhaust treatment technology, in use for high levels of particulate matter reduction, is the diesel particulate filter device (“DPF”). There are several known filter structures used in DPF's that have displayed effectiveness in removing the particulate matter from the exhaust gas such as ceramic honeycomb wall-flow filters, wound or packed fiber filters, open cell foams, sintered metal fibers, etc. Ceramic wall flow filters have experienced significant acceptance in automotive applications. The filter is a physical structure for removing particulates from exhaust gas and, as a result, the accumulation of filtered particulates will have the effect of increasing the exhaust system backpressure experienced by the engine. To address backpressure increases caused by the accumulation of exhaust gas particulates, the DPF is periodically cleaned, or regenerated. Regeneration of a DPF in vehicle applications is typically automatic and is controlled by an engine or other controller based on signals generated by engine and exhaust system sensors. The regeneration event involves increasing the temperature of the DPF to levels that are often above 600° C. in order to burn the accumulated particulates.
One method of generating the temperatures required in the exhaust system for regeneration of the DPF is to deliver unburned HC to an oxidation catalyst device disposed upstream of the DPF. The HC may be delivered by injecting fuel directly into the exhaust gas system typically using a downstream facing exhaust fluid injector. The HC is oxidized in the oxidation catalyst device resulting in an exothermic reaction that raises the temperature of the exhaust gas. The heated exhaust gas travels downstream to the DPF and burns the particulate accumulation.
While systems that employ SCR catalysts and DPFs have been used for NOX and particulate reduction in exhaust gas flow streams, the packaging of the various devices has been problematic, particularly in relatively smaller vehicles having relatively shorter wheelbases, due to the reduced space available to package the desired combinations of devices and the associated injection systems required for the introduction of various exhaust treatment fluids. In some cases there is not enough room to package the catalyst and filter devices while also providing the needed mixing length for conversion of the injected urea into ammonia and vaporization of HC, particularly if the system also employs multiple exhaust treatment devices for the reduction of, or oxidation of, other exhaust constituents, including carbon monoxide (CO), various hydrocarbons (HC), particulate matter (PM) and the like.